The need for polyproyplene homopolymers with excellent stiffness at high flowability is constantly increasing as down-gauging and light-weighing become more important with the need for saving energy resources. High flowability polypropylenes are typically used in moulding and particularly the automotive business where injection moulding is the preferred conversion-process. Especially for glass fibre applications high flow homopolymer with good mechanical properties and high thermal stability is required.
The production of ultra high MFR polypropylene homopolymers may be achieved by controlling the reaction conditions such that a fraction of molecules having relatively short chains is obtained. However, increasing the fraction of molecules having relatively short chains is detrimental to impact strength. Another problem makes a solution even more complex. Higher stiffness of polypropylene is conventionally accompanied by higher shrinkage. Shrinkage, being a measure for three-dimensional stability in moulding, is related with the applicability for moulding applications.
From a process perspective, the provision of polypropylene having a high flowability and simultaneously high stiffness is challenging. Increasing the hydrogen concentration in the polymerization reactor can result in considerably high amount of amorphous material limiting crystallinity and insofar stiffness. A known concept of overcoming these limitations is the use of external donors. External donors increase the hydrogen response and can significantly increase the production rate to crystalline material. However, donors resulting in a better hydrogen response yield materials with lower isotacticity, and insofar lower stiffness.
From a general perspective, stiffness is mainly influenced by the crystal structure which is mainly influenced by chain regularity and further by molecular weight. The crystal structure is inter alia reflected by the lamella thickness being directly measurable by stepwise isothermal segregation technique (SIST). Stepwise isothermal segregation technique (SIST) is a stepwise fractioning by crystallization when cooling the molten sample.
Heterophasic polypropylenes having a matrix phase and a dispersed rubber phase also have stiffness values far below the desired range originating from the dispersed phase. EP 2174980 discloses such materials produced by a triple reactor cascade.
Similar limitations as to stiffness also hold for bimodal polypropylene random copolymers such as disclosed in WO2011/000557.
US 2004/0116629 discloses an ultra high melt flow polypropylene resin obtained by either an internal diether based donor such as Mitsui RK-100 and Mitsui RH-200 or an external donor such cyclohexylmethyl dimethoxysilane (CMDS), dicyclopentyl dimethoxysilane (CPDS), diisopropyl dimethoxysilane (DIDS), cyclohexylisopropyl dimethoxysilane (CIDS), and di-t-butyl dimethoxysilane (DTDS). Though the obtained polypropylenes have relatively high crystallinity, the mechanical properties still need to be improved.
Thus, there is still the need for a polypropylene having high stiffness at a given very high melt flow rate and additionally acceptable good shrinkage values.